Oxygen sensor and process for producing same

Abstract

An oxygen sensor for sensing the oxygen content of an exhaust gas from an automotive engine and a process for producing the same, which sensor is less sensitive to lead poisoning than conventional sensors. In the inventive sensor, a first platinum electrode is formed on the side of a solid electrolyte body exposed to the gas to be detected, and a second electrode is formed on the other side of the electrolyte body. The first electrode comprises a platinum layer and a layer of at least one of rhodium and palladium. Preferably, the layer of rhodium or palladium has a thickness of 10 to 5,000 ANGSTROM .

We claim: 
     
       1. In a process for producing an oxygen sensor having a first electrode formed on that side of a solid electrolyte body which is faced to a gas to be detected and a second electrode formed on the other side of the solid electrolyte body, the improvement comprising the steps of: forming a first electrode metal layer made of a mixture of platinum and at least one of rhodium and palladium directly on that surface of the solid electrolyte body where the first electrode is to be formed, and subjecting the first electrode metal layer to oxidation and then to reduction to allow said rhodium and/or palladium in said mixture to be concentrated and overlaid on said platinum to produce a dual structure in the first electrode of a platinum layer overlaid with a layer of concentrated rhodium and/or palladium.   
     
     
       2. A process according to claim 1, wherein a protective layer is formed on the side of said first electrode exposed to the gas to be detected. 
     
     
       3. A process according to claim 1, wherein said solid electrolyte body is made of at least one material selected from the group consisting of zirconia, thoria, ceria, ceria-lanthania, ceria-thoria-lanthania, and bismuth oxide. 
     
     
       4. A process according to claim 1, wherein said solid electrolyte body is comprised of zirconia doped with an oxide of at least one element selected from the group consisting of yttrium, calcium, magnesium, ytterbium, cerium, scandium, lanthanum, and strontium. 
     
     
       5. A process according to claim 1, wherein said first electrode metal layer is formed by one of spraying, print firing, sputtering, evaporation and plating. 
     
     
       6. A process according to claim 1, wherein the amount of rhodium and/or palladium to be incorporated in said first electrode is in the range of 0.1 to 50 wt% of the total weight of said metal layer. 
     
     
       7. A process according to claim 1, wherein the heating temperature is in the range of 600° to 900° C. and the duration time is in the range of 5 minutes to 3 hours in said oxidation treatment. 
     
     
       8. A process according to claim 1, wherein the heating temperature is in the range of 100° to 200° C. and the duration time is in the range of 5 minutes to 1 hour in said reducing treatment. 
     
     
       9. A process according to claim 1, wherein said first electrode metal layer is heated in oxygen gas and then heated in hydrogen gas to form the first electrode including the rhodium and/or palladium layer with a thickness of 10 to 5000 Å.